Transparent substrate equipped with an electrode

ABSTRACT

The subject of the invention is a transparent substrate, especially made of glass, which is provided with an electrode, especially for a solar cell, comprising a conductive layer based on molybdenum Mo with a thickness of at most 500 nm, especially at most 400 nm or at most 300 nm or at most 200 nm.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. 120 to U.S. applicationSer. No. 10/466,335, filed Mar. 2, 2004, the entire contents of whichare hereby incorporated in its entirety. U.S. application Ser. No.10/466,335 is the National Stage of PCT/FR02/00274, filed Jan. 23, 2002.This application also claims foreign priority to French Application No.01/1292, filed Jan. 31, 2001.

FIELD OF THE INVENTION

The invention relates to a transparent substrate, especially made ofglass, which is provided with an electrode. This conductive substrate ismost particularly intended to form part of solar cells.

BACKGROUND

It is known in fact that solar cells incorporate this type of conductivesubstrate, which is then coated with a layer of absorbent, generallymade of a chalcopyrite based on copper Cu, indium In and selenium Seand/or sulfur S. It may, for example be a material of the CulnSe₂ type.This type of material is known by the abbreviation CIS.

For this type of application, the electrodes are usually based onmolybdenum Mo, as this material has a number of advantages: it is a goodelectrical conductor (relatively low specific resistance of about 5.2mW·cm); it may be subjected to the necessary high heat treatments, as ithas a high melting point (2,610° C.); and it exhibits good resistance,to a certain extent, to selenium and to sulfur. The deposition of theabsorbent layer usually means that there must be contact with anatmosphere containing selenium or sulfur, which tends to cause mostmetals to deteriorate. In contrast, molybdenum reacts on the surfacewith selenium especially, forming MoSe₂. However, it keeps most of itsproperties, especially electrical properties, and maintains a suitableelectrical contact with the CIS layer. Finally, molybdenum is a materialwhich adheres well to the CIS layers—it even tends to promote theircrystalline growth thereof.

However, it has a major drawback when considering industrialproduction—it is an expensive material. This is because the Mo layersare usually deposited by sputtering (enhanced by a magnetic field). Now,Mo targets are expensive. This is all the less negligible as, in orderto obtain the desired level of electrical conductivity (a resistance persquare of less than 2, and preferably less than 1 or 0.5 ohms:square Ω/□after treatment in an atmosphere containing S or Se), a thick Mo layer,generally of about 700 nm to 1 micrometer, is required.

SUMMARY

The object of the invention is therefore to obtain a substrate providedwith an electrode intended for solar cells, which is simpler tomanufacture and/or less expensive than the known Mo electrodes, butwhose performance, especially electrical performance, is equivalent orat the very least sufficient for the envisaged application.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a ×1000 magnification micrograph of the multi-colored coatedglass substrate of Example 1.

FIG. 2 is a ×1000 magnification micrograph of the multi-colored coatedglass substrate of Example 5.

DETAILED DESCRIPTION OF THE INVENTION

The subject of the invention is firstly a transparent substrate,especially made of glass, which is provided with an electrode,especially one suitable for solar cells, and which comprises aconductive layer based on molybdenum Mo of at most 500 nm, especially atmost 400 or at most 300 or at most 200 nm. Preferably, it has athickness of at least 20 nm or at least 50 or 80 or 100 nm.

The subject of the invention is firstly a transparent substrate,especially made of glass, which is provided with an electrode,especially one suitable for solar cells, and which comprises aconductive layer based on molybdenum Mo of at most 500 nm, especially atmost 400 or at most 300 or at most 200 nm. Preferably, it has athickness of at least 20 nm or at least 50 or 80 or 100 nm.

Within the context of the invention, the term “layer” is understood tomean either a continuous layer or a discontinuous layer having, inparticular, patterns (either by etching of a continuous layer or bydirect deposition of the discontinuous layer with the desired pattern,or by a mask system for example). This applies to all the layersinvolved in the present application.

The approach of the invention did not consist in completely eliminatingmolybdenum in favor of another metal, as no metal was deemed to besufficiently resistant, especially when in contact with selenium orsulfur, and during the abovementioned heat treatments, without majordegradation (the problem of possible degradation of the Mo has in thesame way an impact on the absorbent layer which covers it). However, theapproach has been to reduce significantly the molybdenum thickness: ithas turned out, against all expectations, that thicknesses much lessthan those normally used—much less than 1 micrometer—are quitesufficient to obtain the desired electrical properties with, as aconsequence, an appreciable saving in terms of raw materials cost.Decreasing the thickness of the molybdenum layer has another advantage:it has turned out that it is possible for these relatively thin layersto be deposited by sputtering with deposition parameters resulting inhighly stressed layers without the problems of delamination that mayarise in this particular case with thick layers. Thinner layers alsotend to have fewer defects known as pinholes.

To ensure that thinner molybdenum layers maintain their effectiveness,the invention preferably lends itself to several variants (which remain,however, optional) used as alternatives or in combination.

According to a first variant, a layer called a barrier layer isadvantageously inserted between the substrate and the electrode. Itsmain function is to form a barrier to the migration of diffusing speciesout of the substrate into the electrode and as far as the absorbentlayer (and possibly, reciprocally, of diffusing species from theelectrode into the substrate). When the substrate is made of glass, thespecies liable to diffuse out of the glass and degrade the electrode andthe absorbent layer are especially alkali metals. By providing such abarrier layer, it is possible to use as substrate a standardsilica-soda-lime glass obtained by the float process, without any riskof the electrode or the absorbent layer made of chalcopyritedeteriorating. Within the context of the invention, this is all the moreimportant the thinner the molybdenum layer since any degradation evenover a small thickness would have a greater impact than in the case of amuch thicker layer.

Advantageously, this barrier layer is based on a dielectric materialchosen from at least one of the following compounds: silicon nitride oroxynitride, aluminum nitride or oxynitride, silicon oxide or oxynitride.Silicon nitride (possibly containing a minority metal of the Al or borontype) has proved to be particularly effective. This is a very inertmaterial, which is practically insensitive to the heat treatments andprovides a satisfactory barrier to the diffusion of alkali metals.

Preferably, the barrier layer has a thickness of at least 20 nm,especially at least 100 or 120 or 150 nm, and a thickness of at most 300nm, especially at most 250 or 200 nm.

According to a second variant, it may be desirable to “compensate” forthe reduction in thickness of the Mo layer in order to obtain anelectrode which, overall, is as conductive if not more conductive than athick Mo layer. The solution according to the invention consists inadding, in the electrode, to the Mo-based layer at least one otherconductive layer of a different kind. Advantageously, this“complementary” conductive layer(s) is(are) chosen to be made ofmaterials which are less expensive to deposit (by sputtering) as a thinlayer than molybdenum.

The complementary conductive layer, or the combination of thecomplementary conductive layers if there are several of them, preferablyhas a thickness of at least 10 nm, especially at least 40 nm.Preferably, it has a thickness of at most 300 nm and the thickness isadvantageously chosen to be in a range from 50 to 200 or 300 nm.

According to a first embodiment of this second variant, the electrodecomprises at least one complementary conductive layer called M based ona metal or an alloy of at least two metals. These may especially be thefollowing metals or alloys: Cu, Ag, Al, Ta, Ni, Cr, NiCr, steel. It isadvantageous to place this (these) complementary metal layer(s) beneaththe molybdenum-based layer: in this configuration, the molybdenum layerisolates these metal layers from contact with selenium or sulfur, theseelements being particularly corrosive and molybdenum being properlyresistant to them.

According to a second embodiment of the second variant, as analternative to or in conjunction with the first embodiment, theelectrode comprises at least one complementary conductive layer calledM′N based on a metal nitride. This may especially be a nitride of atleast one of the following metals: Ta, Zr, Nb, Ti, Mo, Hf. This layermay be located beneath or above the molybdenum-based layer (or there maybe two layers, one beneath said layer and the other above it). Thenitride may be stoichiometric, substoichiometric or superstoichiometricin terms of nitrogen. The stoichiometry may be adjusted, especially byvarying the percentage of nitrogen in the sputtering chamber when thelayer is deposited by reactive sputtering using a metal target.

One particularly advantageous embodiment consists in combining the firsttwo, by providing a layer M′N between a layer M and the Mo-based layer.This is because in this configuration the nitride layer M′N not onlyacts as a conductive layer but also as a layer preventing (or at thevery least significantly reducing) any interdiffusion of species betweenthe two metal layers (M and Mo). It has turned out that the TiN, TaN,ZrN, NbN and MoN layers were effective for preventing the diffusion ofcopper into the molybdenum layer. It has also been shown that HfN layerswere particularly effective for preventing the diffusion of aluminuminto the molybdenum layer (this type of formulation based on HfN, etc.does not prejudice the stoichiometry of the nitride—it may either be asubstoichiometric or superstoichiometric nitride, as is the case for allthe other nitride formulations mentioned in the present text).

It is possible to have, for example, configurations of electrodesaccording to the invention which comprise the following sequences oflayers:

-   -   M/Mo/M′N    -   M/M′N/Mo    -   M/Mo    -   M′N/Mo    -   Mo/M′N.

If the layer of metal M is based on silver, it is preferable to ensurethat it adheres well to the subjacent layer (for example the Si₃N₄-typebarrier layer in one configuration, namely barrier layer/M layer/M′Nlayer/Mo layer), by inserting, between said barrier layer and saidsilver-based layer, a nucleation layer based on zinc oxide. It may alsobe beneficial, again to ensure better cohesion of the multilayer, toprovide a second layer based on zinc oxide on top of the silver layer.The ZnO-based layer or layers, with the ZnO possibly being doped (withAl, B, etc.), are for example chosen to have a thickness of at least 5nm, for example between 7 and 20 nm.

Preferably, the sum of the thicknesses of the conductive layers of theelectrode is less than or equal to 600 nm, especially less than or equalto 500 or 400 nm.

Advantageously, the electrode has a resistance per square of less thanor equal to 2Ω/□, especially less than or equal to 1Ω/□ and preferablyless than or equal to 0.50 or 0.45Ω/□: these values are appropriate forsolar cell electrodes.

According to a preferred variant, the invention has aimed to improve theappearance of solar cells. This is because when a solar cell is fittedto the walls or roof of a building, its appearance on the “inside” ofthe building (on the outside, the electrode forms a mirror) is notalways very attractive. The colorimetric response in reflection issusceptible to improvement.

A first solution to this subsidiary problem according to the inventionconsists in including the abovementioned barrier layer in a multilayercoating for optical purposes “beneath” the actual electrode. The opticalcoating consists of at least two layers of dielectric materials havingdifferent refractive indices. By varying the thicknesses and the indexdifferences between the layers, the colorimetric response of themultilayer-coated substrate in reflection may thus be quite finelyadjusted, by interference.

Preferably, this coating comprises an alternation of layers having ahigh index (1.9 to 2.3 for example) and layers having a low index (1.4to 1.7 for example). Embodiments of this coating are, for example,Si₃N₄/SiO₂ or Si₃N₄/SiO₂/Si₃N₄.

A second solution, as an alternative to or in conjunction with thefirst, consists in using electrodes containing at least one layer basedon a nitride M′N and in modifying, slightly, the nitrogen stoichiometry.This is because it has been found that slightly substoichiometric orsuperstoicheomtric nitrides retain the same electrical properties butallow the colorimetric response of the substrate to be varied to acertain degree. By combining the two solutions, the number of options inadjusting the colorimetric response is increased.

A third solution, as an alternative or in conjunction with at least oneof the first two solutions, consists in placing a thin layer absorbentin the visible beneath the electrode, especially by interposing itbetween the barrier layer and the electrode. For example, these may belayers of a metal nitride, of the TiN type, and they preferably have athickness lying within the range from 2 to 15 nm. It is thus possible tohave a sequence of layers of the type: glass/barrier layer, such asSi₃N₄/thin absorbent layer, such as TiN/SiO₂/Mo—in this case, theabsorbent layer lies “in the middle” of the Si₃N₄/SiO₂ optical coating.

It is thus possible to obtain a substrate provided with this coating andwith the electrode which, in reflection, has negative a* and b* valuesin the (L,a*,b*) colorimetry system, which corresponds to colors in theblue-greens, or a slightly positive a* value and a negative b* value,which corresponds to a color in the pinks.

The subject of the invention is also the substrate defined above andcoated, on top of the electrode, with a layer of chalcopyrite absorbent.

The subject of the invention is also said substrate for making solarcells.

The invention will now be explained in detail with the aid ofnonlimiting examples of embodiments of electrodes for solar cells,illustrated in FIGS. 1 and 2 by micrographs of the multilayer-coatedglass substrates.

Each example uses a clear silica-soda-lime glass substrate 2 mm inthickness (the glass substrates generally have a thickness of between 1and 4 or between 1 and 3 mm).

All the layers were deposited on the glass substrates bymagnetic-field-enhanced sputtering:

-   -   the metal layers using a target of the corresponding metal, in        an inert atmosphere;    -   the metal nitride layers using a target of the corresponding        metal, in a reactive atmosphere containing a mixture of inert        gas and nitrogen;    -   the silicon nitride layers using an Al-doped Si target, in a        reactive atmosphere containing a mixture of inert gas and        nitrogen; and    -   the silicon oxide layers using an Al-doped Si target and a        reactive atmosphere comprising a mixture of inert gas and        oxygen.

The layers were tested in the following manner:

-   -   {circle around (1)}—measurement of the resistance per square        R□(1) by the four-point method after all the layers have been        deposited;

{circle around (2)}—the so-called “bronze” test: this test consists inheating the glass provided with all the layers at 350° C. for 10 minutesin air. The test is intended to check whether or not sodium has diffusedfrom the glass into the electrode. At the end of the test, theresistance per square, R/□ (2), is measured, again by the four-pointmethod. In addition, a microscope was used (at ×100 and ×1000magnifications) to check whether or not the heat treatment has causeddefects (pinholes), etc.;

{circle around (3)}—the so-called “selenization” test: this testconsists in again heating the glass substrates provided with all thelayers in a selenium atmosphere for 10 minutes. The selenium temperatureis between 200 and 240° C. and the glass temperature between 325 and365° C. At the end of the test, the resistance per square, R/□ (3), isagain measured and from this is calculated the difference inresistivity, ΔR□ (3), between the value before selenization and thevalue after selenization.

It should be noted that this selenization test is in fact much “harder”than in reality. This is because the invention here pertains firstlyonly to the manufacture of the electrodes. However, when the solar cellin its entirety is manufactured, this selenization step takes place oncethe CIS layer has been deposited: in the normal manufacturing cycle fora solar cell, the electrode is “protected” from direct contact withselenium by the chalcopyrite layer.

For the electrodes to be regarded as being satisfactory, it is deemedadvantageous:

-   -   for the sodium in the glass to be prevented from diffusing into        the electrode;    -   for the electrode to have a certain resistance to the “bronze        test” and to the “selenization” test, namely few defects and        sufficient resistance per square;    -   for the electrode to adhere well to the CIS layers; and    -   for the electrode to be easily etchable, especially by a laser.

Examples 1 to 7 Example 1

This example uses a barrier layer and a molybdenum monolayer electrode,in the sequence:

-   -   glass/Si₃N₄ (200 nm)/Mo (500 nm).

FIG. 1 is a ×1000 magnification in a microscope of the glass after theselenization step. The micrograph shows that there are few defects,which are also small. It is considered that the quality of the electrodeis good.

Example 1a

This example uses the same multilayer stack as in example 1, but with amarkedly thinner Mo layer, in the sequence:

-   -   glass/Si₃N₄ (200 nm)/Mo 200 nm).

Example 2

This example uses a barrier layer and a bilayer electrode, namely alayer of metal M then a layer of Mo, in the following sequence:

-   -   glass/Si₃N₄ (200 nm)/Ag (50 nm)/Mo (175 nm).

Example 3

This example uses the same configuration as in example 2, but withanother type of layer of metal M:

-   -   glass/Si₃N₄ (200 nm)/Al (100 nm)/Mo (175 nm).

Example 4

This example uses a barrier layer and a metal/metal nitride/Mo trilayerelectrode in the sequence:

-   -   glass/Si₃N₄ (200 nm)/Cu (100 nm)/TiN (100 nm)/Mo (175 nm).

Example 5

This is the same configuration as example 4, but with a differentthickness for the copper layer:

-   -   glass/Si₃N₄ (200 nm)/Cu (50 nm)/TiN (100 nm)/Mo (175 nm).

FIG. 2 corresponds to a micrograph at ×1000 magnification of thisexample 5 for a portion of the multilayer-coated glass after theselenization test: few defects may be seen, and these are very small.This micrograph is quite comparable to that in FIG. 1.

Example 6

This example uses a barrier layer and a trilayer electrode, in thefollowing sequence:

-   -   glass/Si₃N₄ (200 nm)/Ag (50 nm)/TiN (100 nm)/Mo (175 nm).

It corresponds to example 2, but with in addition a TiN layer.

Example 7

This example again uses a barrier layer and a trilayer electrode, in thefollowing sequence:

-   -   glass/Si₃N₄ (200 nm)/Al (100 nm)/TiN (100 nm)/Mo (175 nm).

It corresponds to example 3, but with in addition a TiN layer.

Table 1 below gives, for each of examples 1 to 7, the values of R┌ (1)and R□ (2), the number of defects after the bronze test (“defects”) andthe value of ΔR□ (3), these terms having been explained above.

TABLE 1 EXAMPLES R□ (1) R□ (2) DEFECTS ΔR□ (3) Ex. 1 0.37 0.37 None 0 to−5%    Si₃N₄/Mo Ex. 1a 0.98 0.96 None 0 to −3%    Si₃N₄/Mo Ex. 2 0.420.42 None −17% Si₃N₄/Ag/Mo Ex. 3 0.36 0.34 None — Si₃N₄/Al/Mo Ex. 4 0.450.45 None  −9% Si₃N₄/Cu/TiN/Mo Ex. 5 0.44 0.44 None −10% Si₃N₄/Cu/TiN/MoEx. 6 0.44 0.44 None −12% Si₃N₄/Ag/TiN/Mo Ex. 7 0.38 0.36 None —Si₃N₄/Al/TiN/Mo

The following conclusions may be drawn from this data:

An R┌ value markedly less than 1 ohm/square may be obtained with lessthan 200 nm of molybdenum, which is associated with a layer of metalnitride and/or of metal having reasonable thicknesses (in total, thebilayer or trilayer electrodes have an overall thickness of less than400 or 500 nm).

The Si₃N₄ barrier layers are effective and prevent the electrode fromdeteriorating by sodium diffusion, since in all the examples the R□ (1)and R□ (2) values are the same or almost the same. They therefore alsoprevent the CIS absorbent layer from deteriorating.

It is also possible to choose to have an Mo monolayer electrode (example1: 500 nm combined with a barrier layer). It gives good results. It iseven possible to have a resistance per square of less than 1 ohm. squarewith an electrode composed of only 200 nm of Mo. This proves that it isunnecessary to have a much thicker Mo layer, as believed to be the casehitherto.

Examples 8 to 11b

The purpose of these examples is to adjust the colorimetric response ofthe electrode in reflection.

The molybdenum layer in all these examples has a thickness of 400 nm or500 nm. From the standpoint of colorimetric response on the glass side,the molybdenum layer has no effect beyond a thickness of at least 50 to100 nm, since it is then a perfectly opaque mirror layer: the resultswould be the same with an Mo layer of 175 or 200 nm.

Example 8

This example uses the following multilayer stack:

-   -   glass/Si₃N₄ (200 nm)/TiN (100 nm)/Mo (400 nm)        the TiN layer being deposited by reactive sputtering in a        reactive atmosphere containing 20% nitrogen by volume.

Example 8a

This is the same configuration as in example 8, but in this case the TiNlayer was deposited in an atmosphere containing 40% nitrogen.

Example 8b

This is the same configuration as in example 8, but in this case the TiNlayer was deposited in an atmosphere containing 70% nitrogen.

The table below gives, for these three examples, values of a* and b* inthe (L,a*,b*) colorimetry system, the values being measured on the glassside, and the R┌ values (measurements made before the “bronze” test).

EXAMPLES A* b* R□ (ohm · square) Ex. 8 −8.6 19.4 0.44 Ex. 8a −9.2 1.50.38 Ex. 8b −11.6 −3.6 0.35

It may be seen that the variation in TiN stoichiometry (which depends onthe amount of N₂ during the deposition) does not significantly modifythe electrical properties of the electrode. On the other hand, it doesallow great modifications in the value of a* and, even more, the valueof b*—example 8 is thus colored in the reds with a very positive b*,whereas example 8c is in the blue-greens, with a slightly negative b*.

Example 8 has a slightly substoichiometric TiN layer, example 8a has anapproximately stoichiometric TiN layer, while example 8c has a tendencyto be superstoichiometric in terms of nitrogen.

Example 9

In this example, the Si₃N₄ barrier layer (with a refractive index ofabout 2) is combined with an additional layer based on SiO₂ (with arefractive index of about 1.45) in order to make a high-index/low-indexoptical coating.

The configuration is as follows:

-   -   glass/Si₃N₄ (200 nm)/SiO₂ (20 nm)/TiN (100 nm)/Mo (400 nm).

The TiN layer was deposited in an atmosphere containing 20% nitrogen byvolume.

Example 9a

Example 9 was repeated, but this time with 40% nitrogen in the TiNdeposition atmosphere.

Example 9b

Example 9 was repeated, but this time with 70% nitrogen in the TiNdeposition atmosphere.

The table below gives, for these 3 examples, the a*, b* and Rsquarevalues explained above.

EXAMPLES a* b* R□ (ohm · square) Ex. 9 −8 .1 22.5 0.34 Ex. 9a −10.6 −8.30.38 Ex. 9b −14.0 5.5 0.35

Example 10

This time, the nitride layer used was made of NbN, in the followingconfiguration:

-   -   glass/Si₃N₄ (200 nm)/SiO₂ (30 nm/NbN (100 nm)/Mo (500 nm).

The NbN layer was deposited in an atmosphere containing 20% nitrogen.

Example 10a

Example 10 was repeated, but in this case the NbN layer was deposited inan atmosphere containing 40% nitrogen.

Example 10b

Example 10 was repeated, but in this case the NbN layer was deposited inan atmosphere containing 70% nitrogen.

The table below gives, for these three examples, the a*, b* and Rsquarevalues already explained:

EXAMPLES a* b* R□ (ohm/square) Ex. 10 −14 −0.5 0.29 Ex. 10a −10.6 −9.20.37 Ex. 10b −17.6 −0.9 0.42

In this case, although the NbN is more substoichiometric orsuperstoichiometric, the a* and b* values are negative, whichcorresponds to an attractive blue-green color whose intensity varies.

Example 11

This example repeats the sequence of layers of examples 10, 10a, 10b,but with different Si₃N₄ and SiO₂ thicknesses.

The configuration is as follows:

-   -   glass/Si₃N₄ (150 nm)/SiO₂ (90 nm)/NbN (100 nm)/Mo (500 nm).

Example 11a

This example repeats example 11, but in this case the NbN layer wasdeposited in an atmosphere containing 40% nitrogen.

Example 11b

This example repeats example 11, but in this case the NbN layer wasdeposited in an atmosphere containing 70% nitrogen.

The table below gives, for these three examples, the a*, b* and R squarevalues already explained:

EXAMPLES a* b* R□ (ohm/square) Ex. 11 0.3 −7.6 0.34 Ex. 11a 2.8 −10.30.33 Ex. 11b 8.8 −14.2 0.28

These examples are therefore in the pinks, this color also being deemedto be attractive.

Example 12

This example has the following sequence of layers:

-   -   glass/Si₃N₄ (150 nm)/SiO₂ (65 nm)/Si₃N₄ (15 nm)/Mo (500 nm).

It therefore incorporates the barrier layer in ahigh-index/low-index/high-index three-layer optical coating.

The table below gives the same data for this example as in the previousexamples.

EXAMPLES a* b* R□ (ohm/square) Ex. 12 −4.1 −6.3 0.28

This example therefore has a color in the blue-greens, also of lowintensity.

In conclusion, the color of the electrodes according to the inventioncan therefore be finely adjusted by varying the stoichiometry of thenitride layer and/or by adding a filter to at least two layersadvantageously including the barrier layer. Moreover, it has beenconfirmed that the a* and b* values of this second series of examplesvary little (by less than ±2) once the “bronze” test has been passed.

1. A substrate comprising: an electrode; a conductive layer includingmolybdenum Mo having a thickness of at most 500 nm; and at least onecomplementary conductive layer M different from the molybdenum-basedlayer, based on a metal or metal alloy, and provided beneath themolybdenum-based conductive layer Mo.
 2. The substrate as claimed inclaim 1, wherein the metal or metal alloy of the at least onecomplementary conductive layer M is selected from one of the followingmetals or alloys: Cu, Ag, Al, Ta, Ni, Cr, NiCr, steel.
 3. The substrateas claimed in claim 1, wherein the molybdenum-based layer has athickness of at least 20 nm.
 4. The substrate as claimed in claim 1,wherein the substrate further comprises: at least one barrier layerwhich provides a barrier against alkali metals, and which is providedbetween said substrate and said electrode.
 5. The substrate as claimedin claim 4, wherein the at least one barrier layer includes a dielectricmaterial chosen from at least one of the following compounds: siliconnitride or oxynitride, aluminum nitride or oxynitride, silicon oxide oroxycarbide.
 6. The substrate as claimed in claim 4, wherein the barrierlayer has a thickness of at least 20 nm, and at most 300 nm.
 7. Thesubstrate as claimed in claim 1, wherein at least one of thecomplementary conductive layers M has a thickness of at least 10 nm, andat most 300 nm.
 8. The substrate as claimed in claim 1, wherein theelectrode comprises at least one complementary conductive layer M′Nincluding a nitride of at least one of the following metals: Ta, Zr, Nb,Ti, Mo, Hf, said nitride being substoichiometric, stoichiometric orsuperstoichio-metric in terms of nitrogen.
 9. The substrate as claimedin claim 8, wherein the layer M′N is provided between the layer M andthe Mo-based layer.
 10. The substrate as claimed in claim 1, wherein thesum of the thicknesses of the conductive layers of the electrode is lessthan or equal to 600 nm.
 11. The substrate as claimed in claim 1,wherein the electrode has a resistance per square R of less than orequal to 2Ω/□.
 12. The substrate as claimed in claim 4, wherein thebarrier layer forms part of a multilayer coating which includes at leasttwo layers of dielectric materials having different refractive indices.13. The substrate as claimed in claim 12, further comprising alternatinglayers having a refractive index, between 1.9 and 2.3, and layers havinga refractive index, between 1.4 and 1.7, in the sequences Si₃N₄/SiO₂ orSi₃N₄/SiO₂/Si₃N₄.
 14. The substrate as claimed in claim 12, wherein thecomposition of the optical coating at least partly regulates thecolorimetric response of the substrate in reflection, in the blue-greenswith negative a* and b* values or in the pinks with slightly positive a*values and negative b* values.
 15. The substrate as claimed in claim 8,wherein the nitrogen stoichiometry of the nitride layer M′N at leastpartly regulates the colorimetric response of the substrate inreflection, in the blue-greens with negative a* and b* values or in thepinks with positive a* values and negative b* values.
 16. The substrateas claimed in claim 15, wherein a thin layer, absorbent in visible lightregion, made of TiN, and having a thickness of 2 to 15 nm, is insertedbetween the barrier layer and the electrode so as to at least partlyadjust the colorimetric response of the substrate in reflection, in theblue-greens with negative a* and b* values or in the pinks with slightlypositive a* values and negative b* values.
 17. The substrate as claimedin claim 1, further comprising a layer of a chalcopyrite absorbent ontop of the electrode.
 18. The substrate as claimed in claim 1, used as asolar cell electrode.
 19. The substrate as claimed in claim 1, used tomake a solar cell.
 20. A solar cell, comprising the substrate as claimedin claim 1.